Time Machine: Shared Drive and Firewire Connection Question

I'm thinking of backing up two of my computers to a shared external firewire drive using Time Machine. However, this summer I'll be working in another state, and would like to bring my external firewire drive with me and be able to do backups while directly connected.

Ideally, I would like to update the existing .sparce image and not have to completely start over when I go to the directly connected backups.

Has anybody used Time Machine to do this? Any Apple Scripts that would allow me to change the path of the backup disk without starting over?